Heart failure is a condition in which the heart is unable to pump enough blood to sustain normal bodily functions. Heart failure may affect either the right side, left side, or both sides of the heart. As pumping action is lost, blood may back up into other areas of the body, including the liver, gastrointestinal tract, and extremities (right-sided heart failure), or the lungs (left-sided heart failure). Structural or functional causes of heart failure include high blood pressure (hypertension), valvular heart disease, congenital heart diseases, cardiomyopathy, heart tumor, and other heart diseases. Precipitating factors include infections with high fever or complicated infections, use of negative inotropic drugs (such as beta-blockers and calcium channel blocker), anemia, irregular heartbeats (arrhythmias), hyperthyroidism, and kidney disease.
Implantable cardiac devices, such as pacemakers and defibrillators, monitor many different cardiac parameters that may be used to determine how well a patient's heart is functioning. For instance, implantable cardiac devices can measure morphology-related parameters, impedance, intrinsic heart rate, heart rate recovery, heart rate variability, conduction delay, pressure, posture, activity, and so forth. Each of these parameters can be used to evaluate the patient's heart.
The implantable cardiac devices are commonly configured to stimulate the heart with pulses in response to individual or combinations of these measured parameters. Additionally, the devices can store these measured parameters over time and periodically transmit the measured parameters to external diagnostic systems for more exhaustive analysis. Unfortunately, in some instances, it can be difficult to derive meaningful values from the measured parameters.
In one such example, the measured values of some parameters are difficult to interpret due to variations caused by other factors, such as bodily functions or patient activity. For example, measured thoracic impedance values decrease when a ratio of fluid to air in a sensed region of the thorax increases. Such a condition occurs when the patient experiences pulmonary edema. However, such a condition also occurs when the patient exhales. It is difficult to interpret measured thoracic impedance values due to the multiple possible factors affecting the measured parameter. Accordingly, there is a need to process individual parameters in a way that assists in diagnosing the patient's cardiac health.